The present invention relates to a weighing module that is equipped with a precisely position-maintaining overload protection device arranged between the weighing cell and the load receiver.
An overload protection device in a balance has the function of uncoupling the load receiver, specifically the weighing pan, from the weighing cell when a maximum permissible load is exceeded, with the force vector of the load acting on the weighing pan in a largely vertical direction. As a result, the weighing cell is protected from the consequences of excessive primarily vertical forces which can be generated either statically by a weight that exceeds a maximum permissible load, or dynamically, for example if a load is set down too rapidly. However, it is also possible for the load receiver to be exposed to transverse forces which in the absence of provisions for uncoupling can be passed on through the force-transmitting linkage to the weighing cell where they can cause damage to the sensitive weighing cell mechanism.
An overload protection device of this kind for a precision balance is disclosed in commonly-owned U.S. Pat. No. 3,973,637, where the weighing pan is configured in two parts. The pan receiving the materials to be weighed lies on a bottom part made of light sheet metal; the bottom part has an outer ring ending only a few millimeters above the housing surface of the balance. Centered in the outer ring, a hollow bolt with a collar resting loosely on the bottom part is engaged with play in a bore hole. A safety ring is imbedded on the shaft of the hollow bolt. Further, a leaf spring of star-shaped configuration is held captive between the safety ring and the bottom part, wherein the leaf spring with the ends of its arms pushes the bottom part upwards against the collar of the bolt. The bias tension of the star-shaped leaf spring is of such a magnitude that the force exerted by the bias tension on the bottom part is larger than the specified maximum load of the precision balance. Consequently, during weighing activities within the weighing range of the balance, the bottom part is held in compressive contact against the collar, while the spring exerts no influence on the weighing result. When an eccentric load is applied to the weighing pan, in particular a load which, in addition to being eccentric, exceeds the weighing range of the balance, the weighing pan can tip on the side of the load and its bottom part comes into contact with the balance housing. As a result, damage to mechanical elements of the weighing cell is avoided, for example to the legs and guide members of the parallelogram mechanism, flexure pivots, etc. This device, which belongs to the known state of the art, meets its purpose as long as the overload acts on the weighing pan substantially in the loading direction. A force that is applied at a right angle, or nearly at a right angle, to the loading direction is not absorbed by the spring-protected support of the weighing pan, but is introduced directly into the force-transmitting linkage and transmitted to the sensitive weighing cell mechanism.
Disclosed in German Gebrauchmuster 297 02 954 U1 is an overload safety device for the weighing pan of a top-loading balance. The device has a pre-tensioned compressive spring which within the permissible weighing range connects the weighing pan in a play-free and quasi-rigid manner to the load receiver of the weighing system and which yields elastically when the permissible weighing range is exceeded. A conically configured compressive contact element and a correspondingly shaped part cooperate to create a connection between weighing pan and load receiver that ensures stability against tipping and is in addition free of play within the permissible weighing range.
In commonly-owned U.S. Pat. No. 5,604,334, a precision balance is disclosed whose load receiver is connected by way of the force-transmitting linkage to the load-receiving portion of the weighing cell with relative mobility between the load receiver and the load-receiving portion. The load receiver spans across the load-receiving potion of the weighing cell like a yoke and rests on the ends of two arms which form the force-transmitting linkage and have guided mobility, pre-tensioned by a spring. Forces that are introduced into the weighing pan eccentrically or at an oblique angle cause a tilting of the load receiver which through its downwardly protruding support legs rests on the force-transmitting linkage that elastically yields in the overload range. However, the disclosed embodiment of this overload protection device can absorb only those transverse forces that are applied from either side, but not from the front or back. When transverse forces are applied from the front or back, the weighing pan is not displaced; rather, the pin connection at the front side of the weighing pan transmits the transverse forces directly to the weighing cell mechanism. As this overload safety device is normally used in balances with a draft-protection arrangement, the loading of the weighing pan occurs from the side. Consequently, one can expect transverse forces to be directed primarily from the sides.
Disclosed in Japanese 2564791 B2 is a force-measuring device which has a weighing cell with a load-receiving portion and a stationary portion, a load receiver, and a force-transmitting linkage arranged between the load receiver and the load-receiving portion. An overload protection device with an elastic element pre-tensioning the load-receiver against the load-receiving portion is integrated in the force-transmitting linkage to protect the weighing cell from being overloaded by static and dynamic forces. Thus, in case of overload and/or with the occurrence of transverse forces, the arrangement of the elements as described in this patent will likewise allow an elastic yielding of a spring device or a guided movement towards a stop that is in fixed connection with the housing. As a first disadvantage of this configuration, with large transverse forces the force-transmitting linkage due to its guided displacement will bear against the housing-based stop and the transverse forces are transmitted to the weighing cell mechanism as tensile or compressive forces. In some applications, a second disadvantage of the disclosed embodiment lies in the lack of a coaxial alignment of the axis of the force-transmitting linkage with the axis of the bore hole located in the load-receiving portion of the weighing cell, surrounding the force-transmitting linkage. Necessarily, the bore hole must have sufficient play relative to the force-transmitting linkage in order to allow a displacement of the force-transmitting linkage in the manner discussed above. As a third disadvantage, this design is not suitable to uncouple the weighing cell from tensile forces that may act on the load receiver against the load direction.
It is often necessary to attach to the load receiver a setup that is adapted to the specific application. To meet the stringent requirements in regard to the reproducibility of a weighing, this setup preferably has a fixed connection to the load receiver. However, in this kind of a situation it will be necessary to protect the weighing cell against damage that may result from torques acting at a right angle to the force being applied, which may be caused, for example, by forces acting tangentially on the load receiver. These kinds of forces occur particularly in cases where the possibility is provided to attach a setup on top of the load receiver with a screw connection, in particular a weighing pan or an adapter. An overload protection device of this kind for a force-measuring device is disclosed in commonly-owned U.S. Pat. No. 6,958,453 B2.
In integral automated production and test systems, it is particularly suitable to incorporate balances of a modular design, so-called weighing modules. This term applies essentially to balances in which the indicator unit is arranged separately from the balance, as for example in a system with a central indicator for a plurality of weighing modules. Integrated weighing modules of this kind are used with preference in systems for the production and testing of small, expensive parts, for example in filling and packaging machines for tablets, ampoules, capsules, etc. in the pharmaceutical industry or in the quality control of ball bearings.
To allow a plurality of parallel weighings of individual masses of weighing objects of the same kind, a solution presents itself in the form of two-dimensional matrix-like array of weighing modules arranged in a compact space. Given that a handling device is used, for example a robot arm with multiple gripping devices, to put these weighing objects in parallel on the individual load receivers of the weighing modules and to remove them again from the load receivers after the weighing process, the positions of the individual load receivers have to be precisely defined in relation to each other and to the handling device. During the weighing activity, the load receivers have to be held without change in their positions at least within a plane that runs orthogonal to the direction of the load. In the case where the load-handling is performed by a robot arm, a malfunction in the handling device can cause an overload, tensile force or transverse load to act on the load receivers in any direction, and this can have an adverse effect on the weighing performance. Among the current state of the art, no solution is known which would have the required configuration for this area of application.
It is therefore an object of the present invention to propose a weighing module whose overload protection device uncouples from the weighing cell at least one overload in the load direction as well as transverse forces orthogonal to the load direction. In addition torques due to the installation or and/or un-installation of load receivers on the force-transmitting linkage should be uncoupled as well. In order to allow the weighing processes to continue immediately after the overload protection device has been activated due to the application of additional forces, it needs to be ensured that the load receiver returns to the original position after the forces have subsided. The weighing module which is mounted on a weighing module support must not change its place relative to the latter in spite of transverse forces occurring on the load receiver. In addition, suitable means are to be provided to facilitate the precise positioning of the weighing module at its location of use prior to installation, to prevent the need for adjustments associated with the installation of the weighing module at the location of use.